This invention relates to a vehicle climate control system, and more particularly to a method of compensating the control for variations in relative ambient humidity.
A vehicle automatic climate control system regulates the operation of a conventional refrigeration cycle with the objective of satisfying a desired cabin air temperature (set temperature) selected by the driver. The system typically controls the blower speed, the evaporator air discharge temperature and the air delivery mode, based on the set temperature, calibration tables and various measured parameters, including the ambient air temperature, solar loading, and the actual cabin air temperature. Another parameter that significantly affects the system operation, but which is typically not measured due to cost considerations. is the relative humidity of the ambient air. Thus, although a climate control system may be calibrated to perform well at nominal ambient humidity, the cooling tends to be excessive at lower-than-nominal ambient humidity and insufficient at higher-than-nominal humidity. This occurs because the amount of work that must be performed for given level of system performance includes not only the work required to reduce the temperature of the air, but also the work required to de-humidify the air. Although a humidity sensor could be used to measure the ambient humidity for purposes of compensating the nominal control settings, the sensor would significantly increase system cost. Accordingly what is needed is a method of compensating an automatic climate control system for variations in the ambient humidity in a cost effective manner.
The present invention is directed to an improved method of control for a vehicle climate control system in which system control settings are adjusted in accordance with an ambient humidity correction value to compensate for variations in relative ambient humidity, and wherein the correction value is determined based on normally measured system operating parameters. In general, the ambient humidity correction value is determined according to a difference between the cooling performance of the system and the ambient heat energy absorbed by the system. According to the invention, the cooling performance of the system is determined during an initial period of vehicle operation based on the time required to reduce the initial evaporator temperature by a predetermined amount. Once steady-state operation of the system is achieved, the cooling performance of the system is determined by the steady-state deviation of the evaporator outlet air temperature from a target temperature of the system. The ambient heat energy absorbed by the system is estimated based on the temperature of the ambient air and the flow rate of air through the evaporator.